App hangs on index synchronization

[gtandersen]

Since the fixes to correct the unread message counters in the folder list, the app hangs for a short while on every index synchronization.

The severity is proportional with the number of messages and/or folders, and in large accounts the app will not react for several seconds on each sync.

Related Github issues:

• Unread message counters are unreliable: #Mail: Unread message counters are unreliable #512
• Fast and correct folder counters: Fast and correct folder counters #513
• Make sure we show folder counts correctly for folders not available in local index: fix(folderlist): Make sure we show folder counts correctly for folders not available in local index #639
• Only recount folder unread counts after content change: fix(webmail): Only recount folder unread counts after content change #653

Possibly related comment: https://community.runbox.com/t/runbox-7-webmail-bugs-that-really-annoy-me/1211/15

[tadzik]

#665 (currently secretly deployed as runbox7-beta) cuts the folder counters down by an order of magnitude, but it may still not be enough especially on slower devices, like phones.

We may want to consider doing that in a separate thread (using https://developer.mozilla.org/en-US/docs/Web/API/Web_Workers_API/Using_web_workers), which should eliminate the hangs even if the operation itself remains slow.

[gtandersen]

There is still some lag on message actions, so performing synchronization in a separate thread sounds like a good idea.

Another idea is to only synchronize folder counts after a period of inactivity (say 10 minutes) and in the meantime calculate counts locally by adding and subtracting upon each message action.

[gtandersen]

This seems better now but very variable. See also #483 (closed).

[gtandersen]

See also internal issue 1156.